Prior art sonobuoys typically provide for the formation of 24 beams, covering 360 degrees of azimuth from 40 hydrophones. The prior art sonobuoys are typically planar arrays of 40 elements which operate over two (2) octaves of frequency. One such sonobuoy is deployed from an ASW aircraft. For such an application, the sonobuoy is deployed in the water where it receives acoustic signals from hydrophones and transmits acoustic data, via an RF link to the aircraft.
A major constraint on the operation of the sonobuoy is the limitation that no more than 256 kbps of data may be transmitted over the present RF link. Further, due to overhead consideration, i.e., Barker codes and compass information, both known in the art, the data rate of the RF link is typically reduced to 195,765 bps. This data rate of 195,765 bps typically includes twenty-four (24) narrower patterns of radiation, commonly referred to as beams, transmitted from the sonobuoy and data from an omnidirectional phone. This limitation also causes a reduction in the acoustic bandwidth of frequencies being transmitted by the sonobuoys, typically resulting about a reduction to 500 Hz from the desired 750 Hz. The reduced bandwidth operation is typically manifested by sonobuoys transmitting one large band of only 500 Hz acoustic information.
This constraint on data rate necessitates a trade-off to be made on the amount of bandwidth and amplitude quantization that can be achieved with 24 beams, wherein quantization is the division of the range of values into finite sub-ranges. With a given array, higher frequency beams are narrower in beam width than lower frequency beams. If only one band is used across a wide frequency band, and the system employed by the sonobuoy is utilized with the upper frequency beams of the selected one band and crossing over at the 3 db operating point, then the lower frequency beams will cross over at less than 3 db, and become redundant. Further, if a beam, employed for a sonobuoy operating over two (2) octaves of frequency, at frequency f1, is 15 degrees wide, then a beam at f1/2 will be 30 degrees wide, favoring the use of beams at the higher frequencies. Typically, the number of beams employed for sonobuoys is determined by the upper frequency of the beams being transmitted. The prior art sonobuoys do not take into account the situation that at the lower end of the frequency beams, the beams are highly overlapping and, thus, not efficiently utilized. It is desired to more effectively utilize the lower end of the frequency band W of the beams transmitted by sonobuoys.